Study Of The Crystalline Mechanism Of LTA And FAU Zeolites With UV-Raman Spectroscopy

A large number of zeolite type materials have been prepared with a variety of properties and chemical compositions during the past decades which can be used as ion-exchangers, sorbents, and catalyst. However, because zeolite nucleation and growth is not well understood at a molecular level, most of the discoveries in zeolite synthesis have evolved in an empirical fashion. It has been proposed that nucleation and the subsequent crystal growth of zeolites proceed through the condensation of silicate and aluminate ions in solution, although the mechanisms underlying this process have not been fully elucidated yet. The technological importance of these materials has resulted in extensive studies on the influence of composition, temperature, mixing procedure, agitation, synthesis time, pH and environment on the zeolitization process, therefore, elucidation of the crystallization mechanism of zeolites remains one of the major pursuits in zeolite science. A basic understanding of the crystallization process at a molecular level coupled with this enormous body of existing empirical knowledge will provide opportunities for synthesis of new zeolites.Various methods such as X-ray diffraction and scattering, solid–state NMR spectroscopy, atomic force microscopy , electron microscopy and Mass spectroscopy have been used to find evidence of different mechanisms. Many salient features of zeolites dynamics and structure can be deduced from vibrational spectroscopic data, such as infrared and Raman spectroscopic which can provide molecular information on the early stage of zeolite formation . Raman scattering studies of aluminosilicate molecular sieves have yielded a great deal of information on the framework vibrations of these materials and have provided empirical correlations between framework vibrational modes(lattice phonons) and the presence of discrete structural .Raman spectra of zeolites are often obscured by a broad fluorescence background, so obtaining zeolite Raman spectra displaying a good signal-to-noise ratio presents a considerable challenge. The intensity of this background, which has been related to the zeolite acid strength, can be minimized with careful sample handling; however, each successive postsynthetic treatment leads to an increased background signa1.Contrast with the Visible Raman the UV Raman gives a considerable good signal-to-noise ratio especially at the 200~500cm-1 region, also can reduce the broad fluorescence background which is wonderful for the insitu observation and the liquid phase study.UV Raman was employed to reveal the crystalline process mechanism of zeolites of LTA, Y and EMT. Both are aluminosilicate molecular sieves that consist of arrangements ofβ-cages(sodalite cages) linked through the six-membered rings by hexagonal prisms. The only difference is that theβ-cages of EMT(hexagonal) linked through two ways while zeolite Y(cubic) only one way. The two structures were chosen because of their similar topology frameworks which facilitates the comparison of the signals from framework vibrations.The role of aging of the reactant mixture was studied. During the nucleation period, the solid amorphous phase consists of predominantly four-membered aluminosilicate rings, which act as building blocks for the formation of zeolite Y and EMT. It is essential to have polymeric, highly condensed silicate units as a reactant if crystallization is to take place.